At present, researchers studying the stability and quality characteristics of automatic control systems and their dynamic components make an increasing use of experimental methods alongside the more conventional analytical methods. Experiments play an especially prominent role in assessing the quality characteristics of an automatic control system prior to putting the latter into operation, since its actual characteristics may differ from estimated performance.
In the course of such experimental checking, the system and its individual components can undergo a whole series of tests, including measurements of frequency amplitude and phase characteristics, the speed of response, the frequency of self-oscillation which may appear, and the electric capacitance and inductance. As a rule, such measurements are accompanied by a high noise level in the signals being measured.
Today tests of this kind involve a great number of different instruments, each of these instruments being intended for measuring only one or two characteristics (for example, only the phase shift or only the harmonic oscillation frequency). In addition, the known instruments have a narrow frequency range, are too sophisticated and bulky and display an extremely low measurement accuracy for non-sinusoidal signals, which shape is typical of non-linear systems. They are also ineffective in cases of a high noise level.
Thus, this field of technology is facing the problem of providing a sufficiently universal measuring instrument which would be capable of operating in a broad frequency range, would ensure high measurement accuracy in the presence of high noise levels and great constant signal components, and, finally, would be reliable and simple to manufacture, adjust and operate.
A device is known in the prior art, for measuring parameters of dynamic objects.
This foregoing prior art device comprises a periodic oscillation generator which generates sinusoidal and non-sinusoidal signals applied to two multipliers that are connected in parallel. One of these signals is applied to the input of a dynamic object being measured. From the output of the dynamic object, the signal is applied via an input converter to second inputs of the multipliers. Connected to the output of each multiplier, via integrating amplifiers, are two indicators. One of the indicators measures the imaginary component, and the other, the actual component of the amplitude-phase frequency characteristics of the dynamic object.
With all its complexity, the foregoing prior art device only makes it possible to measure the imaginary and actual components of amplitude-phase frequency characteristics. To determine the phase shift and amplitudes ratio requires a series of complicated calculations which involve such operations as squaring and root extraction, as well as the use of trigonometric functions. The starting point for these calculations is the value of the imaginary and actual signal components measured by the two indicators. This reduces the accuracy of measurements by 10 to 20 percent. In addition, the measurements take much time.
Apart from the foregoing factors, the presence of constant components in signals being measured reduces the accuracy of measurements, which accuracy is affected by the zero drift of the integrating amplifiers. The inadequate noise immunity of the prior art device in question also accounts for a reduced measurement accuracy. In the course of investigating a dynamic object taken as a whole, it is impossible, by using this device, to study its individual components, which is often necessary in investigating different types of dynamic objects.
A device is also known in the prior art, for measuring the phase shift and amplitude, the device comprising two selsyns with connected stator windings. The rotor of one of the selsyns is set in motion by a variable speed drive including a servo-amplifier and a tachogenerator. The rotor of the driven selsyn is excited from the outside, by the servosystem circuit when investigating an alternating current control system, and by an in-built generator when investigating an alternating current system.
Connected to the shaft of the rotating selsyn is a synchronizing switch which passes a sawtooth signal.
In addition, the foregoing prior art device includes a carrier frequency amplifier, to whose input there is connected the output of the second selsyn, the latter being mechanically coupled to the phase scale of a measuring dial. The device further includes an output stage amplifier connected to an amplitude value indicator. The function of the phase displacement indicator is performed by an oscillograph with vertical and horizontal scanning. A signal from the second selsyn, which is connected to the phase scale and is a phase shifter, is applied to the vertical scanner of the oscillograph, whereas a signal from the first selsyn is applied to the horizontal scanner of the oscillograph. This results in the appearance of a Lissajous figure on the screen of the oscillograph with high-frequency oscillations superimposed thereon. In order to determine the phase shift, the phase scale is rotated together with the movable selsyn until the curve on the screen of the oscillograph corresponds to zero phase shift. Since the phase scale actually shifts the signal by a value equal to and opposite from the phase shift in the system, the phase shift value is found by directly reading the calibrated scale. Thus, the latter prior art device is more effective than the former prior art instrument that has been described above, as the prior art device under review makes it possible to directly measure the phase shift in degrees without resorting to calculations. This device, however, only measures the amplitude value at the output of the system. In order to determine the module, one has to divide the amplitude value at the output of the system by the amplitude value at the input of the system, which brings additional errors and complications into the measuring process. Due to the fact that a selsyn is used as a phase shifter in this device, the latter does not make it possible to measure low-frequency signal in the range below 0.1 Hz, which means that the frequency range of oscillations being measured is limited. Input action in the given device is done mechanically with the aid of a servomotor, which accounts for considerable errors due to variations in the supply voltage. Besides, the device features low noise immunity and does not allow to measure phase shifts between two arbitrary harmonic signals.
Still another device is known in the prior art for measuring parameters of a dynamic object, which comprises a source of periodic oscillations connected to a dynamic object being investigated, whose output is electrically coupled to an indication unit.
This device makes use of Lissajous figures for phase shift measurements. From the output of the dynamic object being investigated and of the periodic oscillations source, signals are applied to the vertical and horizontal plates of an oscillograph which plays the role of an indicator. As this takes place, an ellipse-like image appears on the screen of the oscillograph. By measuring the ellipse parameters and making some additional calculations one can obtain the phase shift values and the amplitude ratios of the signal being measured.
In order to minimize the errors when using the above prior art device, there must be strict equality between the amplification factors at the horizontal and vertical plates of the oscillograph. In addition, the ellipse must be strictly in the center of the cathode ray tube of the oscillograph. It is practically impossible to comply with the two requirements. Besides, this prior art device is rendered inoperative when acted upon by the noise and constant components of the signal applied from the output of the dynamic object. It also takes much time to covert the ellipse parameters into phase shift values and amplitude ratios. The device is highly sensitive to zero drift of the amplifiers and is inapplicable for studies of non-linear objects.